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<TITLE>VRI: Guide to VRI concepts</TITLE>
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<H1><A HREF="../vri.html">V.R.I.  -  Virtual Radio Interferometer</A></H1>
<EM>D. J. McKay  & N. P. F. McKay</EM><P>
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VRI (the Virtual Radio Interferometer) is a Java applet that simulates
the operation of a particular technique in radio interferometry called
<EM>earth rotation aperture synthesis</EM>. To the unititiated, this
can be a quite complicated process, so this document attemtps to
explain some of the fundamental concepts and why the VRI applet is
useful to visualise radio interferometry.

<H2>Radio telescopes</H2>

When people think of astronomy, they often think of the big optical
telescopes, mounted in domes high up on mountain peaks. These
observatories are built to study the visible light that comes from the
stars, galaxies and other bodies that make up the universe. Yet
visible light is just a small part of the <EM>electromagnetic
spectrum</EM>, which also includes other types of radiation, such as
X-rays, ultra-violet (UV) rays, infra-red and radio waves.
<P>
In addition to the optical observatories, other installations have
been contructed to examine these other radiations. We shall be
concentrating on the radio waves here.
<P>
Although there are many different types of radiotelescopes, one of the
more common designs is that of the "big dish" - such as that at <A
HREF="http://wwwpks.atnf.csiro.au/PKSHomePage.html">Parkes</A> (NSW
Austrlia) or <A HREF="http://www.jb.man.ac.uk">Jodrell Bank</A>
(Cheshire, UK). The idea is that radio waves come in from space, are
bounced off the surface of the dish and are focused onto a piece of
electronic equipment - the <EM>receiver</EM>.  This converts the radio
wave into an electrical signal that can be measured.
<P>
These telescopes are huge - many tens of metres across, which raises
the obvious question: why build them so big?

<H2>Sensitivity and resolution</H2>

Radio astronomers want to be able to measure objects in the sky that
can be very small or very faint (often they are both). To measure
faint objects, increasing the collecting area of the dish will
increase the amount of radiation that is focussed onto the
receiver. The ability to discertain small detail in sky is called the
telescope's <EM>resolution</EM> and this is governed by the diameter of
the dish.
<P>
So to build more sensitive telescopes with better resolution,
astronomers simply build bigger dishes - up to a certain limit. Due to
structural limits, one can only build them so big (up to a hundred
metres or so). Certainly dishes of, say, 5 kilometres are out of the
question - especially if you want to be able to steer them around to
point at different parts of the sky.

<H2>Radio interferometry</H2>

Radio interferometry is a way of getting the extra diameter, but
without huge dish sizes. By connecting a number of small dishes
together, astronomers can "simulate" a large dish with the diameter
equal to the largest separation between the <EM>elements</EM>.
<P>
If you just have a row of telescopes, such as the 
<A HREF="/">Australia Telescope Compact Array</A>, the result is a
telescope with great resolution in one direction but poor resolution in
the other. However, by waiting for 6 hours, the earth will have
rotated the telescope with respect to the object in the sky to provide
good resolution in the other direction.
<P>
By plotting the <EM>virtual tracks</Em> that the antennas trace out as
the earth rotates, astronomers can gauge how good the telescope will
be a resolving objects in the sky. This plot is referred to as the
<EM>uv-coverage</EM> as the two axes are U and V (it has nothing to do
with ultra-violet radiation!).
<P>
The uv-coverage is not a mask on the image itself, but is a mask on
the fourier transform of the image. It shows where on the fourier
plane the image has been sampled. It's a bit like putting the
uv-coverage mask over the aperture of an optical telescope.

<H2>An example</H2>

The best way to get a feel for what is going on is to work through
some examples. 
<P>
Start the applet (click on the image to start the applet):

<P><CENTER><A HREF="../vri.html">
<IMG SRC="init_small.gif" ALT="The VRI applet at startup">
</A></CENTER><P>

At the top of the applet is a row of widgets that allow control of
some of the observatory parameters. They are (from left to right): the
observatory (a number of preset telescopes), the latitude of the
observatory (negative latitudes are south of the equator), the number
of antennas/elements, the diameter of each antenna (in metres) and the
minimum elevation limit of the antennas (in degrees above the
horizon).

<P><CENTER>
<IMG SRC="obs.gif" ALT="The VRI observatory parameters">
</CENTER><P>

For out tutorial, set the number of antennas to 3. By clicking on the
<B>Plot</B> button, the resultant uv-coverage will be displayed in the
lower right panel.

<P><CENTER>
<IMG SRC="uv1.gif" ALT="The VRI uv-coverage plot" ALIGN=TOP>
<IMG SRC="plot.gif" ALT="The VRI plot button" ALIGN=TOP>
</CENTER><P>

The white line in the uv-coverage plot shows the scale of the
plot. <EM>Lambda</EM> is the greek letter used to denote
wavelength. 100 klambda is therefore 100,000 wavelengths. If the
wavelength is 10 millimetres, 100 klambda is 1 kilometre. This
means that with the same antenna spacing, doubling the frequency of
the observation doubles the resolution of the telescope. The down side
is that higher frequency observations are harder to make (electronic,
weather and other technical challenges), and at different frequencies,
radio objects in the sky might look different in brightness and shape.
<P>
Using the mouse, try dragging the antennas about on the observatory
layout map. After placing them, click again on the plot button.
<P>

<P><CENTER>
<IMG SRC="ant1.gif" ALT="VRI antenna positions" ALIGN=TOP>
<IMG SRC="ant2.gif" ALT="VRI antenna positions" ALIGN=TOP>
</CENTER><P>

On the "antenna map", the bar at the bottom left shows the scale in
kilometers. On all of the display panels, the zoom buttoms will allow
you to look more closely at the graph, map or images.

<P><CENTER>
<IMG SRC="zoom.gif" ALT="The VRI zoom buttons" ALIGN=TOP>
</CENTER><P>

Also, by placing the mouse cursor over one of the displays, the cursor
keys can be used to scroll the image about. The PgUp and PgDn keys
also zoom the display, to compliment the cursor control.
<P>
The uv-coverage is also affected by where the object is in the sky and
for how long you observe it. VRI can simulate this with <EM>hour
angle</EM> and <EM>declination</EM> control.
<P>
All objects in the sky will rise every day in the east and set in the
west. During that time they will cross an imaginary line called the 
<EM>meridian</EM>, which runs from North to South, passing
overhead. This line is also referred to as having an hour angle of
zero. A full observation usually goes for 12 hours, 6 hours before and
after the source crosses the meridian (i.e. <EM>transit</EM>). VRI can
simulate the different amounts of observing time by altering the
starting and finishing hour angle of the observation.

<P><CENTER>
<IMG SRC="uv1.gif" ALT="VRI antenna positions" ALIGN=TOP><BR>
<IMG SRC="ha1.gif" ALT="VRI hour angle control" ALIGN=TOP>
<P>
<IMG SRC="uv2.gif" ALT="VRI antenna positions" ALIGN=TOP><BR>
<IMG SRC="ha2.gif" ALT="VRI hour angle control" ALIGN=TOP>
</CENTER><P>

The <EM>declination</EM> (celestial latitude) of the radio source will
also affect the resultant uv-coverage.

<P><CENTER>
<IMG SRC="uv1.gif" ALT="VRI antenna positions" ALIGN=TOP><BR>
<IMG SRC="dec1.gif" ALT="VRI declination control" ALIGN=TOP>
<P>
<IMG SRC="uv3.gif" ALT="VRI antenna positions" ALIGN=TOP><BR>
<IMG SRC="dec3.gif" ALT="VRI declination control" ALIGN=TOP>
</CENTER><P>

As you can see, a east-west row of antennas will have better coverage
of a source near the celestial pole than it will of a source near the
equator. Also, because of the minimum elevation limit of the antennas,
it might not be possible to always observe for 6 hours either side of
transit for sources near the equator. Some sources will only be about
the horizon for a short period each day. Try dragging the declination
slider further and further away from the pole and watch the hour angle
sliders shrink in their range.

<H2>Images and transforms</H2>

On the left hand side of the VRI applet, there are two panels for
images and their transforms. Using the source widget, you can select
different objects to examine.

<P><CENTER>
<IMG SRC="source.gif" ALT="VRI image selection" ALIGN=TOP><BR>
</CENTER><P>

On selection, the image will appear in the top left display panel. The
transform buttons <B>FFT</B> and <B>FFT-1</B> (FFT is fast fourier
transform) will convert the image to the transform and vice versa.
Try doing this for different sources. The following is for a
(simulated) radio galaxy. The amplitude of the transform is shown
here.

<P><CENTER>
<IMG SRC="fft.gif" ALT="VRI FFT buttons" ALIGN=CENTER>
<IMG SRC="im1.gif" ALT="VRI image of a source" ALIGN=CENTER>
<IMG SRC="fft1.gif" ALT="VRI fourier transform of a source" ALIGN=CENTER>
</CENTER><P>

Fourier transforms are, in general, arrays of complex numbers; each
pixel having both a phase and amplitude component. The different
representations of the fourier plane can be selected in VRI.

<P><CENTER>
<IMG SRC="disp.gif" ALT="VRI fourier plane display type" ALIGN=TOP>
</CENTER><P>

Using the <B>Apply</B> button, you can blank out the portions of the
uv-plane that were note sampled by the observation of the object. 

<P><CENTER>
<IMG SRC="apply.gif" ALT="VRI uv-coverage apply button" ALIGN=CENTER>
<IMG SRC="im2.gif" ALT="VRI image of a source" ALIGN=CENTER>
<IMG SRC="fft2.gif" ALT="VRI fourier transform of a source" ALIGN=CENTER>
</CENTER><P>

The image (left) indicates how the source would appear if observed
with that particular antenna/time configuration. As you can see, with
just 3 antennas, the image is not very good. To improve it, more
antennas can be used, or the observation repeated with antennas in
different locations. Some observatories have antennas that can be
moved, this allows more complete uv-coverage of an object to be
built-up over time. VRI allows you to select the different
configurations of the ATCA <I>et al.</I>.

<P><CENTER>
<IMG SRC="config.gif" ALT="VRI antenna configuration control" ALIGN=CENTER>
</CENTER><P>

You can simulate the addition of different configurations by clicking
on the <B>Add</B> button after each observation is simulated. When the
<B>Apply</B> button is clicked, all the accumulated observations will
be 

<H2>Experiment!</H2>

The whole idea of the VRI applet is for you, the user, to experiment
with various parameters that go into a radio interferometer
observation. Here are some experiments to try.

<UL>
<LI> Set the hour-angle <EM>range</EM> to zero (this is what is known
     as a "cut"). Accumulate cuts at hour angles of 0, +4 and -4. Look
     at the uv-coverage for 6 antennas and what the resultant image
     looks like for a point source and a more complicated one
     (e.g. the radio galaxy).
<LI> Set the declination to zero. See what happens on an east-west
     array. Now try moving antennas off the E-W line and look at the
     new uv-coverage.
</UL>


<H2>Further information</EM>

<UL>
<LI> <A HREF="../vri.html">V.R.I.</A>  -  the Virtual Radio Interferometer 
     applet itself
<LI> <A HREF="guide.html">VRI Guide</A> - general description of operation
     and concepts for newcomers to radio interferometry
<LI> <A HREF="intro.html">VRI Introduction</A> - a general description of what
     VRI is all about
<LI> <A HREF="doc.html">VRI Documentation</A> - technical notes on running the
     Java applet
</UL>

<H2>Reference</H2>

<OL>
<LI> McKay, D.J., McKay, N.P.F., <CITE>Using Java for Astronomy: The
     Virtual Radio Interferometer Example</CITE>, in preparation, 1997.
</OL>

<H2>Comments?</H2>

We would greatly appreciate any comments you may have on this
documentation, or the VRI java applet.
Please e-mail them to Nuria McKay
(<A HREF="mailto:nm@jb.man.ac.uk"><CODE>nm@jb.man.ac.uk</CODE></A>).

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Return to the
<A HREF="http://www.jb.man.ac.uk">
NRAL - Jodrell Bank</A>
Home page
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Return to the 
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<ADDRESS>
Original: dmckay@atnf.csiro.au (7-MAY-1997)<BR>
Modified: nm@jb.man.ac.uk (10-Sep-1997)
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